1. Field of the Invention
The present invention relates to a thin film of a single crystal of an oxide of the formula: EQU LnA.sub.2 Cu.sub.3 O.sub.7-x (I)
wherein Ln is at least one rare earth element selected from the group consisting of Y, Nd, Sm, Eu, Gd, Dy, Ho, Er, Tm and Yb and A is at least one alkaline earth metal selected from the group consisting of Ba, Sr and Ca which has a three-layered perovskite structure and a process for producing the same.
2. Description of the Related Art
A thin film of LnA.sub.2 Cu.sub.3 O.sub.7-x having a three-layered perovskite structure exhibits superconductivity around 90K will find various new applications such as wiring of LSI, SQUID and Josephson tunnel type elements.
For such applications, the LnA.sub.2 Cu.sub.3 O.sub.7-x thin film should have a critical temperature Tc at 85K or higher, a critical current density Jc of 10.sup.5 A/cm.sup.2 or higher when a film is not thicker than 5,000 .ANG. and should be formed at a comparatively lower temperature.
Since a wiring material of LSI is required to have a large current density, the single crystal thin film should have a specific plane such as a (001), (110) or (103) plane parallel with a film plane so that electric current can flow in said specific plane.
In the Josephson tunnel type element using the superconducting LnA.sub.2 Cu.sub.3 O.sub.7-x, an insulating ultrathin layer between the superconductors for tunnel junction is required to have a thickness of not larger than 30 .ANG.. To form such junction, it is essential to produce a superconductive film having good surface smoothness and an ultrathin insulating layer on it. A thickness of the insulating ultrathin layer for forming a junction is limited by a coherence length of a superconductor. The coherence length in a direction perpendicular to the (001) plane is about 4 to 7 .ANG., and that in a direction parallel to said plane is about 15 to 30 .ANG..
Therefore, the thickness of the insulating ultra-thin layer to be used for junction varies with a kind of the superconductor and its crystal direction to be connected. When a direction perpendicular to the (001) plane of the superconductor coincides with the direction perpendicular to the surface of the insulating layer, the thickness of the latter should be 10 A or less. On the contrary, when a direction parallel with the (001) plane of the superconductor coincides with the direction perpendicular to the surface of the insulating layer, the thickness of the latter can be as thick as several ten .ANG., which makes the formation of tunnel junction easy. Accordingly, a (110) oriented single crystal film may be more available for the tunnel type junction than a (001) oriented film.
In the practical applications, it is required to provide a single crystal film having the (110) plane in a direction parallel with the film plane. In addition, since a single crystal film with an other orientation can achieve a large current density, it is suitable for forming a wire to be used in a superconductive magnet.
In the most cases, the single crystal film of LnA.sub.2 Cu.sub.3 O.sub.7-x has been prepared by a sputtering method. The sputtering method comprises irradiating a plasma of oxygen (O.sub.2) and/or argon gas against a target consisting of a Ln--A--Cu base oxide in a vacuum vessel and depositing sputtered metals and the like on a substrate such as a SrTiO single crystal piece placed in said vessel to form a LnA.sub.2 Cu.sub.3 O.sub.7-x thin film. To covert the deposited LnA.sub.2 Cu.sub.3 O.sub.7-x thin film to a film exhibiting high quality superconductivity with Tc of 77K or higher, it is necessary to thermally treat the film at a temperature of 800.degree. C. or higher.
By the scientists in the Watson Research Laboratory of IBM or the Stanford University, a superconductive oxide thin film is produced by electron beam deposition. But, the as-deposited film is amorphous and does not have superconductive characteristics as such. Therefore, the deposited film is post-heated at a high temperature of 800.degree. to 1,000.degree. C. to crystallize the amorphous film to a perovskite crystal having a three-layered structure, whereby the produced film exhibits superconducting transition at 77K or higher.
By the conventional sputtering method or the conventional electron beam deposition, no film which is substantially a single crystal having the (001), (110) or (103) plane parallel with the film surface has been provided, and either method has its own drawbacks.
For example, in the sputtering method, it is difficult to prepare the target with a composition optimum for the formation of the superconducting film. Since a desired material is deposited solely by attacking the target with ions, not only characteristics of the film are delicately changed according to conditions of a plasma atmosphere and to quality of the target, but also the substrate or the deposited film is easily modified by ions. Therefore, this method has poor reproducibility.
Another problem resides in that the epitaxially grown LnA.sub.2 Cu.sub.3 O.sub.7-x oxide film should be thermally treated at a temperature of 800.degree. C. or higher to increase the critical temperature to 77K or higher, preferably 85K or higher. By the thermal treatment, the surface of the thin film is roughened. Further, due to thermal treatment at high temperature, the substrate material and LnA.sub.2 Cu.sub.3 O.sub.7-x react with each other so that the thin layer having a thickness of 500 .ANG. or less cannot be made superconductive.
A superconductive thin film which is produced by sputtering and subsequent thermal treatment at the lowest temperature is a 2,000 to 3,000 .ANG. thick (001) oriented crystal film of YBa.sub.2 Cu.sub.3 O.sub.7-x which is formed on a sapphire substrate heated at a temperature of 550.degree. to 650.degree. C. It is reported that this film had zero electric resistance at 80K after thermally treated at a temperature of 550.degree. to 650.degree. C.
However, the critical temperature of 80K is still unsatisfactory, and the produced film seems to be heterogeneous from the results of X-ray analysis and change of electrical resistance against temperature.
In the conventional electron beam deposition, the deposited film should be thermally treated at a high temperature. Therefore, a kind of the substrate to be used is limited. When the substrate is inadequate, it reacts with the deposited material so that a part or whole of the deposited material is changed to a material which is different from the superconductor.
Further, a surface of the deposited film is less smooth. Because of the reaction between the substrate and the deposited material, it is difficult to obtain the superconducting film with a thickness of 5,000 .ANG. or less.